1. Field of the Invention
The present invention generally relates to a technical field of mobile communications. More particularly, the present invention relates to a mobile communications system conforming to Mobile IP (Mobile Internet Protocol), and relates to a home agent (HA), a mobile node (MN) and a method that are used for the mobile communications system.
2. Description of the Related Art
As hot spot services are appearing, Internet access is becoming available everywhere in towns. The hot spot service is a service for providing an area where wireless accesses can be performed mostly based on a 802.11x wireless LAN standard. In a hot spot, when a mobile node (mobile terminal) of a subscriber of the hot spot service accesses a wireless router, an address that is valid in the wireless LAN segment is issued to the mobile node by DHCP (Dynamic Host Configuration Protocol) so that IP communications can be performed. There are also charge-free hot spot services.
By the way, when the mobile node moves to another hot spot while performing IP communications, the IP address assigned in the previous wireless LAN segment by the DHCP becomes invalid so that any IP packet does not reach the mobile node. Thus, the communication is interrupted.
As a scheme for solving such problem, there is a protocol called Mobile IP. For the Mobile IP, a following document (to be referred to as non-patent document 1) can be referred to, for example.                [Non-patent document 1] Yuuji Fukunaga, “Part. 3 Examine the mechanism of Mobile IP technically”, searched on Jun. 22, 2006, (http://www.atmarkit.co.jp/fnetwork/tokusyuu/15mobil eip/mip03.html)        
When a process for opening a server port is executed in a mobile node, it is necessary that a home address (HoA) used in a home agent (HA) is a global address. The global address is an address that is uniquely assigned in a wide area network such as the Internet. The global address is distinguished from a care of address that is uniquely assigned only in a particular subnetwork and distinguished from a private address that is uniquely assigned only in an organization and the like. For example, when a mobile node (an IP phone terminal, for example) communicates with another terminal (that can be referred to as CN (Correspondent Node)) using a SIP (Session Initiation Protocol), it is necessary that the mobile node opens a server side RTP port and waits for a connection for performing communications from the correspondent node to the mobile node. In this case, the mobile node needs to have a home address (HoA) that is a global IP address for the correspondent node on the Internet to connect to the server port. Other than the VoIP, when using a game or P2P file exchanging software, the mobile node needs to open the server port so that a global address is necessary as a home address.
FIG. 1 shows a conventional system in which a home address that is a global address is set for each terminal. In this example, a first mobile node MN1 has a first home address HoA1, and a second mobile node MN2 has a second home address HoA2. The home agent manages correspondences between mobile nodes and addresses.
However, the global address is issued by an authorized provider, and certain processes are necessary for obtaining it. In addition, after obtaining the addresses, cost of maintenance is required for the addresses. When global addresses are assigned to each of a plurality of terminals, processes for obtaining addresses and maintenance costs are required for the number of the terminals. Therefore, when many terminals are used by employees in a company or in an organization, the company or the organization needs to prepare global addresses the number of which is the same as the number of the terminals. But, burden for the processes for obtaining the addresses and maintenance costs is not small.
FIG. 2 shows another conventional example. In the example shown in FIG. 2, a home agent is provided in an intra-enterprise network, and the intra-enterprise network is connected to a provider network using a NAT (Network Address Translation) router. A private address uniquely defined in the intra-enterprise network is assigned to each of mobile nodes belonging to the company. Since the private address validly functions only in the company, the private address cannot be used as a global address as it is. The GW/NAT router is a gateway (GW) including a function for converting the private address used in the intra-enterprise network into a global address. Accordingly, irrespective of subnetworks where the mobile node resides, the mobile node can communicate with an external correspondent node by passing through the home agent and the GW/NAT router. Since only one global address is necessary to access the intra-enterprise network in principle, it is not necessary to prepare global addresses the number of which is the same as the number of the terminals as described in the example shown in FIG. 1.
However, according to this method, since private addresses of mobile nodes are managed by the home agent, it is not possible to directly communicate with a correspondent node of a foreign network. More particularly, when the mobile node directly sends a packet to the correspondent node, a private address is set to be a source address of the packet. Thus, the private address is set to be a destination address of a packet sent from the correspondent node to the mobile node (to the home agent), so that the packet cannot reach the mobile node since any route in the Internet cannot be found for the packet. As to a communication from the home agent to the correspondent node, the packet needs to be routed via the gateway. The GW/NAT router derives a global address each time when conversion of a private address is requested by the mobile node side. Thus, in a state in which the request has not been made, the global address cannot be specified. That is, it is not possible that the correspondent node starts to perform a communication with the mobile node.
In addition, according to this method, as to a communication between the mobile node and the home agent, the mobile node needs to establish a tunnel (VPN tunnel) that is a virtual private network from the foreign network to the intra-enterprise network. Therefore, the communication route often results in redundant, and also the overhead becomes large due to encapsulation for VPN and for mobile IP transfer.
In addition, when the gateway converts a packet between the home agent and the correspondent node with NAPT (Network Address Port Translation), it is not possible to support every server process. In a VoIP (Voice over Internet Protocol) communication, a server port to which the correspondent node connects in a RTP session is written into a SIP packet. There is a SIP-NAT that can read out this field to set a NAPT table. However, when server ports included in SIP messages sent by each mobile node are duplicated, only one session can be registered in the NAPT table, so that there may occur a problem in that the other mobile nodes cannot receive a connection from the correspondent node.
A technique related to the above-mentioned method on Mobile IPv4 is described in the following non-patent document 2, and a technique related to the above-mentioned method on Mobile IPv6 is described in the following non-patent document 3.                [Non-patent document 2] IETF (Internet Engineering Task Force), RFC (Request for Comments) 3220, searched on Jun. 22, 2006, (http://www.ietf.org/rfc/rfc3220.txt?number=3220)        [Non-patent document 3] IETF (Internet Engineering Task Force), RFC (Request for Comments) 3775, searched on Jun. 22, 2006, (http://www.ietf.org/rfc/rfc3775.txt?number=3775)        
From the viewpoints mentioned above, it is desired to avoid using such redundant route, and it is desired that, even when a plurality of mobile nodes open server ports, a correspondent node can connect to any mobile node. In addition, it is desired to reduce cost for obtaining plural global addresses to cost for obtaining one global address.